Hello again. Well I got a problem... I am looking at the reaction which transforms succinyl CoA into succinate... catalyzed by succinate thiokinase. There is a molecule H20 that is not included in the scheme at the book. My professor said that there is a lot of water and hydrogen in the cell and that is why it is not included. If it is so, why do the cell need NAD.H as a special transporter of hydrogen. Well id u can enlighten the matter a little bit I would be greatful.

The hydrogen that is available as water is not in an energetically favorable position to provide reducing power; while hydrogen bonded to NAD is. This is because when a water molecule looses a hydrogen it produces a less stable hydroxide ion. While the loose of hydrogen from NADH produces are more stable NAD+ molecule.

 

Note the the hydrogen removed from NADH is also accompanied by the loss of two elections. If this was to happen with Water you'd be left with an ultra unstable species HO+

Thank u very much !